Microbial or biogenic gas is typically generated in anaerobic, sulfate-free sediments at low temperatures (usually less than 75° C.) by a community of microbes that include fermentative bacteria, acetogenic bacteria, and a group of Archaea called methanogens. Methanogens can produce methane by either carbon dioxide reduction (CO2+4H2→CH4+2H2O) or acetate fermentation (CH3COOH→CH4+CO2) with the former pathway being far more common in marine settings and the latter more common in fresh water settings. Although microbial methane is ubiquitous in marine and fresh water sediments, economically recoverable accumulations of microbial gas are less common than thermogenic gas accumulations and require a combination of favorable geological and biological conditions.
Biogenic gas systems, specifically the gas generation mechanisms, differ in many aspects from thermogenic hydrocarbon systems. For example, the timing of microbial gas generation is not controlled by the burial history and thermal cracking kinetics of kerogen but by the timing of the development of optimal living conditions (temperature, nutrient, and pore water chemistry) of methanogens. Early stage methanogenesis (also called primary methanogenesis) begins soon after deposition of sediments, and late-stage methanogenesis (also called secondary methanogenesis) occurs later in geologic time in sedimentary rocks inoculated with methanogens and nutrients by meteoric groundwater. Because of the differences between biogenic gas systems and thermogenic hydrocarbon systems, the approaches for assessing generated hydrocarbon volumes need to be process specific for biogenic and thermogenic systems. Therefore, models specific to biogenic hydrocarbon volume generation need to capture the complexity of biological systems while also accounting for geological conditions. That is, models should adhere to biological and geochemical conditions that determine the feasibility of the microorganisms to produce biogenic gas, and thus should account for microbial kinetic reactions and thermodynamic conditions that provide available free energy. These conditions include temperature, pressure, the concentrations of reactants, such as CO2 and H2, and concentrations of products, such as CH4.
Previous approaches to modeling biogenic gas production have used the total organic carbon or volume of the biogenic gas producing region. Such models directly convert organic matter, both in its bulk organic carbon concentration and in compositional stoichiometric quantities, and, therefore, are not constrained by kinetics or thermodynamics. As such, these organic-matter driven models often fail to accurately predict the volumes of biogenically produced gas as they do not integrate any microbiological component.
Examples of two existing models are those proposed by Clayton (1992) and the kinetics of organic matter degradation published by Wallman et al. (2006). See Clayton, C., (1992) Source volumetrics of biogenic gas generation. In: Vially, R., (ed.) Bacterial Gas. Editions Technip: Paris, pp. 191-204; and Wallman, K., Aloisi, G., Haeckel, M., Obzhirov, A., Pavlova, G., and Tischchenko, P. (2006) Kinetics of organic matter degradation, microbial methane generation, and gas hydrate formation in anoxic marine sediments. Geochimica et Cosmochimica Acta, 70, pp. 3905-3927. The Clayton (1992) approach integrates the drainage or fetch area, which is a geometric area that can produce a hydrocarbon, and the bulk total organic carbon pool to predict a volume of biogenic gas. The Clayton model uses a factor of 10% of the total available organic carbon to calculate the generated volume of biogenic gas regardless of the geochemical conditions in the fetch area which may impact microbial activity. The general characterization of 10% transformation of organic carbon to methane assumes that microbial generation is constant everywhere which is not likely given that geochemical conditions are variable in different environments. These differences could be the presence of products and reactants or temperature, all of which change the kinetics and thermodynamics of microbial methanogenesis. As such, the Clayton model often fails to accurately predict the volume of biogenic gas that has been generated.
The Wallman et al. (2006) model is driven by the overall degradation of organic matter in marine sediments. This approach focuses on the microbial activity at shallow sediment depths in these environments and not necessarily in the deeper depths of the sediments where the temporally protracted generation of biogenic methane is important for the generation of commercially viable volumes of biogenic gas. Accordingly, the model of Wallman et al. (2006) may not be directly transferable to deeper biogenic gas producing areas. In addition, the model of Wallman et al. (2006) requires data that is not often available in exploration settings, such as data detailing the pore-water chemistry sampled at high-resolution over great depths, and dissolved concentrations of CH4, SO42−, and H2, as well as other inorganic parameters. As such, the Wallman et al. (2006) model often fails to accurately predict the volume of biogenic gas that has been generated.
Therefore, the majority of existing models suffer from deficiencies in that they do not integrate the known environmental conditions with microbial activity and energy requirements over geologic timescales. The absence of these parameters limits the ability of these models to predict biogenic gas generation over geological timescales. Therefore, there remains a need for the ability to accurately predict the volume of generated biogenic gas, which is important in assessing and exploring biogenic hydrocarbon systems. An exemplary embodiment of the present invention will more accurately model the conditions responsible for methanogenesis over geological timescales.
Background references may include: Chukwuma Nmegbu, “Modeling the Kinetics of Biogenic Gas Production During Microbial Enhanced Oil Recovery,” International Journal of Scientific and Engineering Research, Vol. 5, Issue 6, June 2014; Barry J. Katz, “Biogenic Gas—Its Formation and Economic Significance”, Proceedings Indonesian Petroleum Association, 24th Annual Convention, October 1995, IPA 95-1.3-222; Barry J. Katz, “Microbial Processes and Natural Gas Accumulations”. The Open Geology Journal, Vol. 5, pp. 75-83 (2011); and U.S. Patent Application Publication Nos. 2010/0155078, 2011/0308790, 2012/0309098, 2014/0163883, 2015/0066461, and 2015/0104795.